Ⅰ.Scaling phenomenon
If the system is shut down without timely flushing, the antiscalant is underdosed or fails to be dosed, or the system recovery rate is excessively high, the water on the membrane surface will keep concentrating. Sparingly soluble salts such as calcium-magnesium carbonate, sulfate and silicate will reach supersaturation and precipitate as scale, clogging the membrane flow channels and causing a continuous rise in system operating differential pressure.
Prevention & Control Measures
Carry out standardized low-pressure flushing when equipment shuts down; calibrate the antiscalant dosing unit regularly; control the system recovery rate reasonably according to raw water quality.
Ⅱ.Various Fouling Phenomena
External impurities adhere to the membrane surface and cause blockage. This type of fouling falls into four main categories, which is different from degradation and damage of the membrane itself.
Inorganic precipitates: inorganic salt crystals such as calcium carbonate, barium sulfate and silica scale;
Colloidal fouling: sediment, iron and aluminum oxides, residual colloids from overdosed flocculants;
Organic fouling: natural humic acid in water, lubricating oil and accumulated excessive antiscalants;
Biological fouling: slime formed by the propagation of bacteria, algae and fungi in feed water.
Prevention & Control
Ensure the feed water SDI ≤ 5; perform regular acid and alkaline chemical cleaning; stabilize the pretreatment process; periodically shock-dose non-oxidizing biocides to inhibit microorganisms and prevent chemical residue.
Ⅲ.Concentration Polarization
Concentration polarization is an inherent physical effect during reverse osmosis operation rather than a fault. However, excessive polarization will trigger various operational problems.
After water molecules pass through the membrane, salts are rejected and accumulate on the membrane surface, forming a high-concentration boundary layer with far higher salinity than the bulk flow. Such a high-salinity environment easily generates salt crystal nuclei and accelerates scaling. Meanwhile, it raises osmotic pressure, increases operating energy consumption, and facilitates microbial reproduction.
Prevention & Control Measures
Maintain sufficient cross-flow scouring velocity on the surface of membrane elements; moderately reduce the system recovery rate; flush membrane elements with high flow regularly to break up the high-concentration concentrate layer.
Ⅳ.Oxidative Degradation phenomenon
The desalination layer of polyamide RO membranes suffers irreversible oxidative damage, while cellulose acetate membranes resist chlorine yet are vulnerable to pH fluctuations.
Oxidants remaining in feed water, such as residual chlorine, chlorine dioxide, ozone and hydrogen peroxide, corrode the membrane desalination layer catalyzed by heavy metal ions like iron and copper. This leads to a sharp drop in salt rejection, and the membrane performance cannot be restored via cleaning.
Prevention & Control Measures
Strictly maintain zero residual chlorine in RO feed water; separate pipelines for disinfection and chemical cleaning from the membrane system during operation; regularly inspect the operating status of activated carbon and sodium bisulfite reduction devices.
Ⅴ.Compaction phenomenon
The membrane separation layer is permanently compacted by external force, which is a type of irreversible aging damage. Typical symptoms include continuous decline in water production under the same operating pressure, accompanied by a slight short-term rise in salt rejection. Severe long-term compaction will cause flow channel collapse and simultaneous deterioration of desalination performance.
Causes: Long-term operation beyond rated pressure, excessive feed water temperature, water hammer generated by pump startup with air trapped in the system, and frequent rapid start-stop of the high-pressure pump.
Prevention & Control Measures
Strictly follow the membrane manufacturer’s specifications for operating pressure and water temperature; purge all air from the system before starting the high-pressure pump; equip the high-pressure pump with variable frequency soft start. Compacted membrane elements cannot be repaired; they can only be replaced or supplemented with new elements to compensate for water production capacity.
Ⅵ.Telescoping phenomenon
It refers to a typical mechanical deformation damage, which occurs more easily in 8-inch large-diameter membrane elements. The outer wrap of the membrane element slides and displaces downstream along the water flow, and the membrane sheets slide relative to the central pipe, resulting in an inward concave end and an outward convex end, shaped like a retractable telescope. Slight displacement will increase the operating differential pressure; severe cases may tear the membrane bonding lines and rupture the membrane sheets.
Causes: No stress-resistant support rings installed inside the pressure vessel, excessively rapid pressure rise of the high-pressure pump, insufficient pressure-bearing rigidity of the pressure vessel, and rough handling during membrane element disassembly and installation.
Prevention & Control Measures
Equip 8-inch membranes with dedicated stress-resistant rings; adopt soft start for high-pressure pumps; select qualified pressure vessels with reliable pressure resistance; standardize the procedures for loading and unloading membrane elements. Deformed membranes cannot be repaired and shall be replaced directly.
Ⅶ.Membrane Rupture phenomenon
Membrane rupture is mostly caused by excessive backpressure on the permeate side. Blocked permeate pipelines or mistakenly closed valves lead to reverse extrusion of membrane elements by concentrate-side pressure, tearing membrane bags and damaging central tubes. In addition, failed pretreatment allows fine sand and carbon powder to enter the pressure vessel and scour the membrane at high velocity; physical collision during assembly and disassembly can also damage membrane surfaces. The most obvious symptom is a sudden sharp rise in permeate conductivity.
Prevention & Control Measures
Install rupture discs on the permeate side for pressure relief protection to avoid over-limit backpressure; strictly follow the operating sequence: open the permeate valve first, then start the high-pressure pump; maintain stable filtration performance of the pretreatment system to prevent hard particles from entering the pressure vessel.
Conclusion
The above covers the seven widely recognized abnormal phenomena of reverse osmosis membranes in the industry, which fall into three major categories:
Restorable via routine operation and maintenance: Scaling, various types of impurity fouling and concentration polarization. These issues can be mitigated through standardized flushing, chemical cleaning and adjustment of operating parameters.
Irreversible material damage: Oxidative degradation and compaction. The inherent membrane structure suffers permanent damage and cannot be restored by cleaning.
Irreversible mechanical damage: Telescoping deformation and membrane rupture. Damaged membrane elements of this kind must be replaced directly.
Regular monitoring of differential pressure, permeate flow and salt rejection during daily operation and maintenance enables early prediction of membrane abnormalities. This practice can effectively extend membrane service life and cut equipment operating costs.
